1. Field of the Invention
The present invention relates to polymer mixtures comprised of poly-.alpha.-methylstyrene and polyvinyl methyl ether which exhibit lower critical solution temperature (LCST) behavior.
2. Description of the Background
Mixtures of polyvinyl methyl ether (PVMe) and polystyrene are among the best-studied "polymer blend" systems. They are frequently referred to as illustrations of the physical chemical model for explaining LCST behavior (see Nishi, T., et al., 1975 Macromolecules, 8 227; Kirk-Othmer, 1982, "Encyclopedia of chemical technology", Third Ed., "Vol 18, pub John Wiley, 443447; Ullmann's Encyclopedia of Industrial Chemistry, 1992, 5th Ed., Vol. 21A, pub. VCH, pp. 273-304). However, thus far, miscible (compatible) polymer systems are the exception. As noted in Ullmann's, ibid., Vol. 20A, 653:
"Plastic blends are usually immiscible because the combinatorial entropy of mixing is too small (high molar masses) and the enthalpy charges on mixing are often positive. Miscibility is observed for three types of systems:
1) Chemically similar systems, e.g. polystyrene-poly(o-chlorostyrene), showing both LCST and UCST. PA2 2) Systems having specific interactions between different components, e.g. polystyrene-poly(vinyl methyl ether); these systems show only LCST. PA2 3) Systems consisting of oligomers, e.g. oligo(ethylene oxide)-oligo (propylene oxide); these systems possess only UCST."
The existence of homogeneous, compatible polymer mixtures is demonstrated by, among other things, the occurrence of the phenomenon of LCST. The LCST phenomenon, which has a theoretical basis, consists of the fact that the miscibility of two nonidentical polymers decreases with increasing temperature. The LCST is thus experimentally determinable (see Olabisi, O., Robeson, L. M., and Shaw, M. T., "Polymer-polymer miscibility" pub Academic Press).
Neutron scattering and light scattering experiments have been used in the study of compatible polymer systems. (See Fischer, E. W., 1987 Mater. Res. Soc. Symp. Proc., 79 73-86, Chem.Abstr., 107, (20):176911q).
In the mid-1980s, technical applications of the LCST behavior of compatible polymer mixtures were proposed. EP 0,177,063 (cf. U.S. Pat. No. 4,722,595) employs such compatible polymer mixtures for recording, storing, and displaying optically readable information on a support. Optically detectable transitions from one phase into another under the influence of thermal energy (or energy forms directly convertible into thermal energy).
EP 0,181,485 (U.S. Pat. No. 4,722,506) provides window panel systems having temperature-dependent light permeability which are made of plastics. These window panel systems consist of a polymer mixture formed from at least two different polymer components and displaying an LCST&lt;150.degree. C. At temperatures below the LCST the mixtures exhibit a single phase, transparent mixture, while above the LCST the polymer components undergo demixing or phase separation. The indices of refraction of the polymer components must be different to provide the temperature dependant light-permeability effects.
The above European patents provide extensive information on, such details as rules for selection, preferred polymer combinations and technical application guidelines.
Nonetheless, the possible technical applications of the LCST phenomenon remain unexploited because of the basic constraints of incompatibility and unpredictability of properties in individual cases. For example, in contrast to polystyrene, poly-p-methylstyrene has been found to be completely incompatible with PVMe.
While the above-noted European patent applications disclose compatible polymer systems along with numerous application possibilities which are quite advantageous, the systems disclosed are still deficient in that there is no single embodiment which can optimally cover all of the application areas and desired temperature ranges. Further, in order to technically exploit these polymer systems not only must the LCST behavior be a reversible process, but the polymers themselves must meet criteria which are somewhat restrictive. In particular, in the area of thermotropic window panes (EP 0,181,485), and in the area of the recording, storage, and display of optically readable information (EP 0,101,485), sharp phase transitions within the shortest possible time are very desirable (EP 0,177,063).